In the production of curable silicone rubber compositions, it is common practice to load the composition, irrespective of being the one-package type or the two-package type, with a hydrophilic silica or a hydrophobic silica having its surface treated with, for example, hexamethyldisilazane in order to ensure satisfactory exertion of properties such as surface smoothness and rubber strength after curing.
However, the affinity of the hydrophilic silica with silicone oil is poor, so that aggregates of fillers such as the silica occur in the cured silicone rubber compositions. Rubbers with excellent properties cannot be obtained by curing of these curable silicone rubber compositions. On the other hand, the affinity of the hydrophobic silica with silicone oil is so high that aggregation in the compositions is scarce. Silica is relatively satisfactorily dispersed therein, and the thixotropy thereof is relatively satisfactory. Thus, it is expected that the compositions, even when applied to, for example, a vertical surface, can form a thick coating film by one-time application thereof. However, these compositions have high viscosity and accordingly exhibit poor sprayability. Further, these compositions, when diluted with a solvent, rapidly lose thixotropy to thereby cause such problems that the coating film suffers from sagging and that the smoothness of the coating film is deteriorated.
For example, first, Japanese Patent Laid-open Publication No. 10(1998)-316933 discloses a paint composition comprising, as principal components, a room temperature curable silicone rubber having a number average molecular weight of 20,000 to 100,000 (a), a room temperature curable silicone rubber having a number average molecular weight of 500 to 20,000 (b) and a silicone oil (c), and also discloses a paint composition further loaded with a hydrophobic fumed silica (d). This paint composition characterized by loading two types of silicone rubbers differing in molecular weight with a hydrophobic fumed silica, although can form a thick coating film by one-time application, poses the following problems. That is, the storage stability of the paint composition is poor. The paint composition, during the preparation and storage, may be gelatinized probably due to the secondary aggregation of silica, so that the sprayability thereof is poor at the time of application. At the formation of coating film, the surface of the coating film becomes nonuniform (film thickness and tone are nonuniform, and coating marks are observable), with the result that a coating film which is excellent in film thickness, rubber strength, etc. cannot be obtained.
Apart from the above, for example, underwater structures and fishing nets are used in water, especially sea water, for a prolonged period of time. Thus, when a multiplicity of sea lives, such as hydrozoans, Bugula, sea lettuces, green layers, serpulas and oysters, adhere to parts in contact with sea water and breed there, there is the danger of deterioration of the inherent functions of the underwater structures and fishing nets. In particular, underwater structures, such as supply water inlets and waste water outlets, for thermal power plants and nuclear power plants are constructed and fixed at given positions in the sea, and culture nets and stationary nets are placed in the sea for a prolonged period of time. Therefore, they are likely to suffer from erosion due to the sea lives and to suffer from conspicuous breeding of the sea lives, so that frequent replacement and cleaning thereof are inevitable. These cause a serious economic loss.
In order to resolve the above problems, it is common practice to apply an antifouling paint to the surface of underwater structures, fishing nets, sea water utilizing apparatus, etc. so as to prevent the sticking of sea lives.
For example, secondly, Japanese Patent Publication No. 63(1988)-2995 discloses a nonpoisonous antifouling paint composition obtained by mixing together a silicone rubber curable by chemical reaction, a petrolatum or liquid paraffin mixture and a low-viscosity silicone oil. In this reference, it is described that application of the paint composition to parts of undersea structure brought into contact with sea water enables preventing sticking and growth of sea lives for a prolonged period of time. However, with the use of this nonpoisonous antifouling paint composition, it is difficult to obtain a satisfactory film thickness by one-time application. Further, there is the problem that the obtained coating film is poor in strength and hardness and is hence likely to suffer from scratching.
Moreover, thirdly, in Japanese Patent No. 2,522,854 granted on the invention formerly proposed by the present applicants, there is disclosed a curable organopolysiloxane composition, together with a product curing thereof, comprising as principal components (A) an organopolysiloxane having its both molecular chain terminals blocked by a silanol group or a hydrolyzable group, (B) an organosilane represented by the formula: R1aSiX4-a (wherein R1 represents an unsubstituted or substituted monovalent hydrocarbon group having 1 to 8 carbon atoms; X represents a hydrolyzable group; and a is 0 or 1), or a partial hydrolyzate of the organosilane, and (C) an organopolysiloxane containing, per molecule, at least one group of the formula: ≡SiR2OSiR3bY3-b (wherein R2 represents an unsubstituted or substituted divalent hydrocarbon group or a divalent hydrocarbon group containing an ether bond; R3 represents an unsubstituted or substituted monovalent hydrocarbon group; Y represents a hydrolyzable group; and b is 0, 1 or 2, provided that, when R2 represents a divalent hydrocarbon group containing an ether bond, an organopolysiloxane wherein two or more trimethylsiloxy groups are bonded to the Si atom to which a carbon atom of the hydrocarbon group is directly bonded is excluded). Further, it is described in the patent that the organopolysiloxane composition can be loaded with, as fillers, not only particulate silica, fumed silica, precipitated silica and the like but also those whose surface has been rendered hydrophobic with the use of, for example, a silane. Still further, it is described in the patent that the cured is free from the danger of environmental pollution and that the antifouling effect persists for a prolonged period of time. However, with respect to the curable organopolysiloxane composition described in the patent, not only is there room for improvement in the thickness increase, strength, hardness, etc. of the coating film but also the problem of poor resistance to scratching is posed.
Moreover, the antifouling paint composition described in the first reference Japanese Patent Laid-open Publication No. 10(1998)-316933, although can form a thick coating film by one-time application as aforementioned, has poor storage stability in the use as a paint, exhibits poor sprayability at the time of coating and causes the coating film to have a nonuniform surface (film thickness and tone are nonuniform, and coating marks are observable) at the time of coating film formation. Thus, desirable coating films cannot be formed therefrom, and, especially in the use as an antifouling coating film, the problem that the coating film is poor in long-term antifouling properties is posed.